our earth has had impact events that no bunker would save us from. like the one that created the moon or the much smaller impact that created the Borealis Basin on mars would boil the oceans and melt the surface.

It would be massively cheaper and faster to robotically colonize near-Earth space and get really, really good at killer asteroid detection and redirection.

Ok but redirection capability must be abundant enough that outright sabotage and terrorism can be fixed by another nation or else you will have an increase in extinction risk.

Th early solar system was very different with way more debris including large planetismals. The planetismals caused the big impacts you mentioned, and the smaller stuff caused the impacts can see on other bodies.

The solar system is much cleaner place now. All the planetismals and most of the asteroids have impacted or been kicked out. Big things are in stable orbits. There are a lot of dangerous asteroids but we track most of the large ones. There is a risk that something big will be kicked out of orbit but it is rare enough we don’t know how unlikely.

Large impacts, 5km or bigger, are every 20 million years.

Yeah you can come to different conclusions than colonizing Mars being a good strategy for human survival. I'm just answering the "why not the desert?" question.